It is of utmost important to have drugs at hand for the treatment of (i) smallpox, (ii) complications of vaccination against smallpox or (iii) monkeypox in humans. Currently only cidofovir, an acyclic nucleoside phosphonate (ANP), designed as an anti-herpes virus drug and used for the treatment of CMV retinitis, would be available for the treatment of poxvirus infections. The aims of this study are to design and synthesize ANPs that are even more potent and selective than cidofovir, that can be administered orally, and that have an excellent safety profile. We will focus on two structural types of parent compounds that we showed to be active in vitro and in vivo against poxviruses: (a) analogs of cidofovir and (b) "open-ring" analogues of HPMP-DAP. Modification of these lead structures should result in novel compounds with an enhanced anti-poxvirus activity and/or improved pharmacological parameters. Modification of cidofovir will take place at the cytosine base (substitution at the N-4 and C-5 positions), whereas in the "open-ring" series modifications will predominantly focus on the alteration of the 6-alkoxy chain bearing the structural features of the HPMP type (modifications of the alkyl chain, phosphonoalkoxy group, and hydroxymethyl group). The compounds will be prepared by the earlier developed methods for the synthesis of ANPs and will be evaluated for their inhibitory effect on the replication of various poxviruses. Compounds with an excellent safety profile, and that are, in addition, equally or more effective than cidofovir will be evaluated in various animal models for vaccinia virus infection. Compounds that prove in these animal models at least as effective as cidofovir will then be the subjects of detailed pharmacological, pharmacokinetic and toxicological studies. In addition prodrug will be prepared to allow excellent activity following oral administration. The final goal of this application is to advance a compound for the treatment of (systemic or cutaneous) poxvirus infections to the initial stages of development, i.e., selecting an IND candidate.